The background of this invention relates to packing sulphur bearing foods such as shrimp, albacore tuna, regular tuna, crab pieces, shredded crab, salmon and shredded chicken. In particular, these products are light in color and need protection from discoloration and contamination which results when the sulphur in them forms sulfides with the ferrous and stannous metal of the container. Since such food products have been packed in containers for many years, it was well known to use zinc oxide containing enamel for coating the plate from which 3-piece cans were formed. This worked well for a number of years because 3-piece cans were fashioned from flat blanks of sheet stock that were rolled into a cylindrical form and along a longitudinal side were sealed by soldering, by bonding, by cementing or by welding. Such flat strips could be heavily precoated and the finished cylinder needed a side seam stripe in order to protect the inside. The formed cylinder had two open ends and to complete the can a bottom and top were added and these were also heavily precoated. It was also known to add aluminum pigmentation to the coating to improve the inside appearance of the container. Such pigmentation was used to mask underfilm staining, and unevenness in the inside appearance by forming a uniform overall coloring. The aluminum pigment is also beneficial in connection with underfilm sulfide staining which is chemical reaction, that cannot be completely prevented, occurring by the migration of sulfides and mercaptans through the film. The aluminum pigmentation permits a certain amount of staining without a visible change in the overall uniform interior coloration. It was also known to use zinc oxide in the coating since sulfides react with zinc oxide forming light colored zinc sulfides rather than forming the black ferrous sulfide on the can or in the product.
The formulation of a coating system which has the advantages of aluminum pigmentation and zinc oxide and is able to be drawn or multiply drawn for purposes of forming a 2-piece container was beset with a number of difficulties. More particularly, coatings which draw such as vinyl organosols can be applied to a flat plate intended to be drawn and redrawn, but such coating material does not readily hold zinc oxide powder in suspension as it causes rapid gellation of the coating. Ordinarily, zinc oxide containing enamels which were acceptable in connection with the coating of 3-piece cans are incapable of accepting the stretching and bending during drawing. They tend to flake off and to lose adhesion during drawing or subsequent processing of foods. It became apparent that a proper coating system containing the aluminum pigment and the zinc oxide that would be able to be drawn and redrawn was needed.
Therefore, an object of the present invention is to provide a coating system which can be used to contain light colored sulphur containing foods, in order to protect them from the discoloration by the drawn containers having ferrous and stannous metals.
It is another object of the invention to provide a coating system which will adhere to the metal substrate during drawing, redrawing and subsequent processing.
It is still a further object of the invention to provide a coating system for drawing a container which will mask underfilm staining; and provide low metal exposure on drawn cans, and thereby minimize black sulfide buildup at coating discontinuities.
Yet a further object of the invention is to provide a coating system which will contain zinc oxide in a manner which permits it to act sacrificially to prevent formation of dark colored metal sulfides and hydrogen sulfide gas when a ferrous drawn container is packed with sulphur bearing foods. The foregoing objects are obtained with the coating system disclosed in the Summary of the Invention which follows.